1. Field of the Invention
The present invention relates generally to a zirconium oxide dental implant manufactured by molding a green body and sintering the green body, especially to a mold for molding the green body for sintering to be a zirconium oxide dental implant with at least an external thread, internal thread and polygonal tapering section.
2. Description of the Related Art
As being illustrated in FIG. 1, the typical use of ordinary dental implant 1 is for connecting a dental crown cap 32 to alveolar bone 2 through an abutment 3. Some products of the dental implant 1 may be formed with an internal thread 12 for securing the abutment 3 by securing screw 31.
The majority of the implants valuable for osseous tissues are made of titanium as pure as possible. These titanium implants 1 are generally tightly fitted in a drill hole at the alveolar bone 2, each formed with an external thread 11 on the peripheral so as to be screwed into the alveolar bone 2, this gives a better security in the quality of the fixing.
Since a few years, new implants are under experimentation. Some of them have been adopted successfully like those made of zirconium oxide with a particularly good robustness linked to the hardness of the zirconium oxide material and consequently a longer life.
Zirconium oxide implants available today may have sometimes a central axial conical hole, the shape of which is polygonal, more often square or hexagonal. Their hole is obtained by molding and then sintering and they can receive prosthetics with a similar shape which have to be adjusted in the well before fixing with dental cement.
However, even with these techniques, the longevity of these zirconium oxide implants is not optimal. In fact, due to the extreme hardness of the material, it is not possible to drill correctly this threaded hole once the implant has been molded. A better solution has provided by U.S. Pat. No. 5,846,136 as being illustrated in FIG. 2, in which a green body 10 can be manufactured by an insert holding mechanism 7, a first molding part 71, a second molding part 72, and a plurality of metal inserts 7c, 7d. The external thread 11 of the green body 10 is formed in a mold cavity between the molding parts 71, 72. An internal thread 12 and a polygonal tapering section are formed by the metal inserts 7c, 7d both of which have to be unscrewed by specific tools from the green body 10 after molded. The metal insert 7c has a rounded shoulder for detachably engaging with a metal ball 7a biased by a spring 7b, and this can be discharged by pulling downward from the insert holding mechanism 7, when the green body 10 is molded in the mold cavity.
Since the direction of the external thread 11 has a thread angle θ and not be possible parallel to the opening direction 75, 76 of the molding part 71, 72, the opening of the mold cavity shall unavoidably and slightly tear off and distort the external thread 11 at the area 73 near the parting line 74. Due to the green body 10 is highly breakable before sintered in high temperature, the operation of unscrewing the metal inserts 7c, 7d and the opening of the mold cavity may cause the green body 10 broken or fracture from time to time in mass production.